The U.S. Senate Foreign Relations Committee has begun ratification hearings on the New Strategic Arms Reduction Treaty - or START. Our correspondent looks at a key element of the treaty - the verification provisions.

The New START Treaty, recently signed by Washington and Moscow, replaces the 1991 START I accord that expired last December.

The New START Treaty sets a limit of 1,550 deployed strategic - or long-range - nuclear warheads. It also sets a limit of 700 operationally deployed strategic nuclear delivery systems such as long-range launchers and heavy bombers.

The accord also provides for what the Obama administration calls strong verification measures - provisions that ensure each side complies with its treaty obligations.

During a recent appearance before the Senate Foreign Relations Committee, Admiral Michael Mullen, Chairman of the Joint Chiefs of Staff, described some of the specific verification provisions.

"For the first time to be able to look into and see the number of weapons that are on top of any particular missile, where we haven't been able to do that before," said Admiral Mullen. "We will be able to count weapons on bombers which we haven't been able to do before. We will be able to, in fact, confirm [military] facility elimination - there are very robust national technical means provisions in this treaty and a specific provision which does not permit interference with that. The unique identifier which will be on every single weapon is a brand new provision for verification."

But former Secretary of State James Baker, who negotiated much of the 1991 START I treaty, described to senators his view of the new provisions.

"This verification regime is nowhere near as intrusive and extensive as what we negotiated in START ," said James Baker. "It's my understanding that we only have 27 Russian nuclear facilities that we have to inspect under this treaty, whereas under START I we had 73."

In her appearance before the Senate Foreign Relations Committee, current Secretary of State Hillary Clinton addressed verification.

"We spent a lot of time on the inspections issue," said Hillary Clinton. "And I have to confess, at first, I wasn't quite sure what the numbers were. But then one of our very able negotiators showed me a map of all the sites in the former Soviet Union that we were inspecting. And then thanks to Senator [Richard] Lugar and other efforts, those sites have been closed, they have been shrunk, they have been dismantled because it wasn't just in Russia - it was in Kazakhstan and Belarus and other places."

Experts say today there are simply fewer Russian military installations to inspect.

Daryl Kimball is Executive Director of the Arms Control Association, a private research firm:

"A superficial analysis of the two treaties' verification regimes might lead somebody to believe that the New START Treaty is not as robust as the old START Treaty," said Daryl Kimball. "But the reality is that verification systems are developed to monitor compliance with the limits that the treaty establishes. The START I treaty had a variety of sub-limits that required a variety of additional information exchanges and inspections. If we look at what is required to monitor this New START Treaty, we see a verification system that is modern, it's more focused on the task at hand and it's more than sufficient to make sure that each side is doing what they say they are supposed to be doing."

Frank Miller, former senior official on the National Security Council under President George W. Bush puts it this way:

"Verification is not a means in and of itself," said Frank Miller. "Verification is a mechanism by which to ensure that the treaty terms are being complied with. And so the verification regime, in any treaty, is tailored to the specifics of the treaty. It is not as intrusive as START I's verification regime but it doesn't have to be given the terms of the treaty to which it is applied."

The Senate Foreign Relations Committee continues its hearings on the New START Treaty ahead of a full Senate ratification vote, expected later this year. Experts say lawmakers will closely scrutinize the verification provisions as they decide whether the new pact serves U.S. interests.